Why does dear old Clark age like a normal human being up until his 20s-30s and then he goes into super-slow aging Kal-El ?
No reason why he shouldn't. Aging isn't a singular process. Put simply, there are two processes at work in the body, cell growth/regeneration and cell damage/death. In the first phase of life, growth dominates and we develop and mature. In the middle phase, the processes are balanced and we stay roughly the same. In the final phase, decay outraces growth and repair and we deteriorate unto death.
Since a Kryptonian under a yellow sun has nigh-magical regenerative abilities, that would cancel out the decay processes and cause the middle phase of life to continue indefinitely. But there's no reason it would have any effect on the first phase of growth and development (except maybe to make it go even faster, followed by much slower or nonexistent adult aging).